IF you have an Android / IOS phone that has the stupid and pointless Facebook app on it, even if you never use the godawful site ... if you allow your Apps to update automatically, or you just manually update without checking ... you may wish to think on this .....
The latest update to "Facebook for Android" (and it applies to IOS as well) changes the built in permissions of the app quite severely .. and includes permission to ..
"read phone state and identity
Allows the app to access the phone features of the device. An app with this permission can determine the phone number and serial number of this phone, whether a call is active, the number that call is connected to and the like."
Another permission allows the app send this information out ..

have a read here .. under permissions ...
https://play.google.com/store/apps/details?id=com.facebook.katanaMany smartphones do not allow these built-in apps to be uninstalled - unfortunately - but on most you can disable the app after a bit of searching....